This invention relates to coherent imaging systems including, for example, radar, sonar, seismic, and ultrasound systems, using vibratory energy, and in particular, but not limited to, phased array ultrasound imaging systems for scan formats such as, by way of example only, linear, steered linear, sector, circular, Vector.RTM., steered Vector.RTM. in imaging modes such as, by way of example only, B-mode (gray-scale) imaging mode and color Doppler imaging mode. In particular, this invention describes post-beamformation signal processing adjustments. Although the invention will be discussed with respect to an ultrasound system, the invention can be implemented with other types of coherent imaging systems.
Beam-to-beam coherency is not required in most prior art ultrasonic imaging systems, although the need for channel-to-channel coherency in phased array imaging systems is well understood. In prior art systems, a requirement for image uniformity is that the amplitude response of the system to a point target at any range on any scan line be substantially identical to the amplitude response to the same target at the same range on an adjacent scan line. The additional requirement for beam-to-beam coherency further implies the phase response (jointly represented with the amplitude response by, for example, an in-phase and quadrature (I/Q) response) of the system to a point target at any range on any scan line also to be substantially identical to the phase response to the same target at the same range on an adjacent scan line. Systematic phase variations can arise in some scan formats. For example, if the apertures associated with successive transmit and receive beams change relative to each other, systematic scan-line-to-scan-line phase variations can be introduced. Likewise, systematic scan-line-to-scan-line phase variations can be introduced if the center frequencies of successive transmit and receive beams change relative to each other. This method requires range-dependent and scan-line-dependent phase correction or adjustments of such systematic variations. Such phase corrections or adjustments may be predetermined and stored in memory to be applied by a complex multiplier to the acquired coherent samples prior to further coherent operations, such as synthesizing new coherent samples as disclosed in co-pending patent application: METHOD AND APPARATUS FOR COHERENT IMAGE FORMATION which is incorporated herein by reference in its entirety.
As further discussed in co-pending U.S. Patent Application entitled METHOD AND APPARATUS FOR ADJUSTABLE FREQUENCY SCANNING IN ULTRASOUND IMAGING, it is also desirable to adjust for systematic phase variations to establish coherent phase alignment among pre-detected beams in a scan caused-by differences in beam-to-beam transmit/receive frequencies by remodulating prior to detection. This is most efficiently performed on the beamformed baseband I/Q signals.
Ultrasound imaging is accomplished by firing (transmitting) into body tissue or other objects to be imaged a scan sequence of focused ultrasonic beams centered along straight lines in space called transmit scan lines. The transmit scan lines are generated by a transmit beamformer and an ultrasound transducer array. The transmit scan lines are spaced to produce a planar linear, planar sector or other display of the tissue via a pre-defined firing or scanning pattern. Focused to some defined depth in the tissue, the ultrasonic transmit continuous-wave (CW) or pulse-wave (PW) signal, propagating at an assumed constant propagation velocity of nominally c=1540 m/sec through the tissue, interacts with the tissue and reflects a small portion of the signal back to the ultrasound transducer array that initiated the ultrasound signal. The round trip delay time is shortest for those targets closest to the ultrasound transducer array, and longest for those targets farthest from the transducer array. With the application of appropriate time delays, the receive beamformer can dynamically focus receive beams along straight lines in space called receive scan lines commencing, for example, with the shallowest range (depth) of interest and evolving toward the deepest range of interest.
Analog and hybrid (analog-digital) phased array beamformer systems are available in the known art. For example, phase array beamformer systems can be found in the following patents which are incorporated herein by reference.
______________________________________ U.S. Pat. No.: Title: Inventor(s): ______________________________________ 4,140,022 MULTIPLE Samuel H. Maslak TRANSDUCER ACOUSTIC IMAGING APPARATUS 4,550,607 PHASED ARRAY Samuel H. Maslak ACOUSTIC IMAGING J. Nelson Wright SYSTEM 4,699,009 DYNAMICALLY Samuel H. Maslak FOCUSED LINEAR Hugh G. Larsen PHASED ARRAY ACOUSTIC IMAGING SYSTEM 5,014,710 STEERED LINEAR Samuel H. Maslak and COLOR DOPPLER Donald J. Burch 5,165,413 IMAGING J. Nelson Wright Hugh G. Larsen Donald R. Langdon Joel S. Chaffin Grant Flash, III ______________________________________
Digital receive beamformer systems have also been proposed in the art with respect to ultrasound systems. By way of example, the following U.S. patents, discuss various aspects of such systems. The patents include:
______________________________________ U.S. Pat. No.: Title: Inventor(s): ______________________________________ 4,809,184 METHOD AND Matthew O'Donnell APPARATUS FOR Mark Magrane FULLY DIGITAL BEAM FORMATION IN A PHASED ARRAY COHERENT IMAGING SYSTEM 4,839,652 METHOD AND Matthew O'Donnell APPARATUS FOR HIGH William E. Engeler SPEED DIGITAL Thomas L. Vogelsong PHASED ARRAY Steven G. Karr COHERENT IMAGING Sharbel E. Noujaim SYSTEM 4,886,069 METHOD OF, AND Matthew O'Donnell APPARATUS FOR, OBTAINING A PLURALITY OF DIFFERENT RETURN ENERGY IMAGING BEAMS RESPONSIVE TO A SINGLE EXCITATION EVENT 4,893,284 CALIBRATION OF Mark G. Magrane PHASED ARRAY ULTRASOUND PROBE 4,896,287 CORDIC COMPLEX Matthew O'Donnell MULTIPLIER William E. Engeler 4,975,885 DIGITAL INPUT STAGE Dietrich Hassler FOR AN ULTRASOUND Erhard Schmidt APPARATUS Peter Wegener 4,983,970 METHOD AND Matthew O'Donnell APPARATUS FOR William E. Engeler DIGITAL PHASED John J. Bloomer ARRAY IMAGING John T. Pedicone 5,005,419 METHOD AND Matthew O'Donnell APPARATUS FOR Kenneth B. Welles, II COHERENT IMAGING Carl R. Crawford SYSTEM Norbert J. Plec Steven G. Karr 5,111,695 DYNAMIC PHASE William E. Engeler FOCUS FOR COHERENT Matthew O'Donnell IMAGING BEAM John T. Pedicone FORMATION John J. Bloomer 5,142,649 ULTRASONIC IMAGING Matthew O'Donnell SYSTEM WITH MULTIPLE, DYNAMICALLY FOCUSED TRANSMIT BEAMS 5,230,340 ULTRASOUND Theador L. Rhyne IMAGING SYSTEM WITH IMPROVED DYNAMIC FOCUSING 5,235,982 DYNAMIC TRANSMIT Matthew O'Donnell FOCUSING OF A STEERED ULTRASONIC BEAM 5,249,578 ULTRASOUND Sidney M. Karp IMAGING SYSTEM Raymond A. Beaudin USING FINITE IMPULSE RESPONSE DIGITAL CLUTTER FILTER WITH FORWARD AND REVERSE COEFFICIENTS ______________________________________
The basic feature of a digital receive beamformer system as disclosed above can include: (1) amplification of the ultrasound signal received at each element of an array such as, for example, a linear array; (2) direct per channel analog-to-digital conversion of the ultrasound signal with an analog-to-digital sampling rate at least twice the highest frequency in the signal; (3) a digital memory to provide delays for focusing; and (4) digital summation of the focused signals from all the channels. Other processing features of a receive beamformer system can include phase rotation of a receive signal on a channel-by-channel basis to provide fine focusing, amplitude scaling (apodization) to control the beam sidelobes, and digital filtering to control the bandwidth of the signal. This art points out the ever present desire to achieve, in an efficient manner, a reconstructed image of high quality.